Citric acid is commercially produced by a culture fermentation process which employs molasses or a sugar such as glucose, sucrose or converted lactose as a substrate and a fungus such as Aspergillus niger or yeast such as Candida lipolytica as biocatalyst. The fermentation product typically contains biomass from the spent microorganism, carbohydrates, amino acids, proteins and salts as well as citric acid which must be separated from the fermentation broth to provide a pure product. Citric acid salts, particularly the sodium salt, are suitable for use as chelators, flavor enhancers and buffers in pharmaceutical, food and industrial applications where a higher pH than citric acid is required. These salts are typically prepared by neutralizing a citric acid solution with a base containing the appropriate cation, e.g. NaOH.
It has now been found that citric acid or citric acid salts may be prepared from partially purified fermentation broth by spray granulation.
U.S. Pat. No. 5,045,459, commonly assigned herein, discloses an improved method for preparing a particulate citric acid material. The citric acid is prepared by the fermentation of an appropriate carbon and hydrogen source as substrate in the presence of an appropriate microorganism to produce a fermentation broth containing citric acid together with impurities including the biomass residue of the microorganism. The fermentation broth is then treated to substantially remove the biomass and thereby provide a partially purified product containing citric acid and from about 2% to 30% (w/w) unreacted substrate and other impurities based on the weight of citric acid. This partially purified product is introduced, without further purification, into the chamber of a fluidized bed reactor where it is suspended in a flow of upwardly rising air together with seed particles to thereby from granules of citric acid. The seed particles may be citric acid, malic acid or fumaric acid as well as other suitable substances.
The literature describes many other techniques for the purification of impure fermentation broth. Among the literature references is published European Patent Application 167,957 owned by Hoechst AG which discloses a process for isolating anhydrous acidic compounds by bringing a solution of the acid into contact with a weakly basic, adsorbant, ion exchange resin, preferably those containing tertiary amino groups, and then desorbing the acid with water and/or steam.
In Offenlegungsschrift DE 3502924, owned by Benckiser GmbH, a citric acid purification process involving membrane filtration, preferably ultrafiltration, together with adsorption of impurities on a non-ionic resin such as polystyrene or polyacrylamide and crystallization is described.
In U.S. Pat. No. 4,851,573 there is disclosed a method for separation of citric acid from its fermentation broth by contacting the broth with a water-insoluble macroreticular gel of a weakly basic anionic exchange resin possessing tertiary amine functional groups or pyridine functional groups and a cross-linked acrylic or styrene resin matrix. The citric acid is desorbed by water or dilute sulfuric acid.
European Patentschrift 163,836 discloses a process for the continuous preparation of granules having a narrow particle size distribution which involves spraying a slurry/solution of the material to be granulated into a fluidized bed by means of a spraying nozzle to form granules. Fine materials escaping from the fluidized bed with the off-gas are separated and returned to the fluidized bed as nuclei for further granule formation and the granules of the desired size are formed by adjusting the sifting gas stream. The finished granules are discharged via one or more countercurrent gravity sifters installed in the inflow plate of the fluidized bed apparatus. Among the materials which are disclosed as capable of being granulated in this manner is citric acid (Zitronensaure).
U.S. Pat. No. 4,275,234 to Baniel et al describes a method for the recovery of organic acids from their aqueous solution by an extraction process comprising a first extraction stage at which the aqueous solution is contacted with a water-immiscible extractant comprising a water-immiscible organic solvent and, dissolved therein, at least one secondary or tertiary amine in which the aggregate number of carbon atoms is at least 20 together with a back extraction step in which the organic extract, separated from the original aqueous solution, is stripped with an aqueous liquid at a temperature which is higher by at least 20.degree. C. than the temperature of the first extraction step.
Other methods used to purify citric acid fermentation broths include ion exchange, nanofiltration and the so-called lime/sulfuric method, i.e. where citric acid is recovered by the addition of a suitable source of calcium ion, e.g. calcium hydroxide (lime), to form the slightly soluble calcium salts of citric acid. This precipitate is further purified by treating it with sulfuric acid to yield calcium sulfate (gypsum) and a solution of free acid.
Regardless of the preliminary purification steps used in the manufacture of citric acid, the final step in its production typically includes a series of crystallization steps involving evaporation, crystallization and centrifugation equipment. A conventional crystallization scheme consists of a batch vacuum-pan evaporator or a forced circulating evaporator coupled with auxiliary tankage and appropriate centrifuge equipment. Within these systems the crystals formed are separated from the mother liquor and advanced to the next crystallization step which can be carried out in a batchwise or continuous operation.
JP 58-49335 claims a method for preparing a granular citric. However, the method entails fusing anhydrous or monohydrate citric acid at 100 to 130 degrees centigrade and mixing the fused citric acid with preheated or unheated calcium carbonate, cooling the mixture to harden and crushing the solidifying product into particles. The object of JP 58-49335 is to provide a calcium preparation with enhanced absorption due to the inclusion of citric acid and the product obtained is a crushed powder as opposed to the particular, granular product obtained with the method of the invention disclosed herein.
It has been found that with some additional purification of the fermentation broth prior to spray granulation, the method of this invention is suitable for preparing a grade of citric acid or sodium citrate suitable for use in foods or drugs.
U.S. Pat. No. 5,104,799, commonly assigned herein, is directed to a method of production of a alkali metal salt of citric acid from partially purified fermentation broth. The citric acid and salts thereof produced from such partially purified fermentation broth are well suited for inclusion in detergent formulations and have the advantages of forming granules which are freer flowing and less inclined to fracture than is citric acid or its salts prepared by crystallization techniques.
With the discovery that phosphates, which had commonly been used in detergent formulations, were detrimentally affecting the environment, citric acid and citrates have been used increasingly. Detergent builders such as tartaric monosuccinate (TMS) and tartaric disuccinate (TDS) have been incorporated. Tartaric monosuccinate and tartaric disuccinate are commonly prepared and used together and will often be referred to herein as TMS/TDS. Analysis of currently marketed dry detergent formulations indicates that this builder combination is not used in dry detergents. It is believed that this lack of use is due to problems in formulating a dry composition since TMS/TDS tends to form a sticky mass that is difficult to recrystallize.
There are many references, both literature and patent, on detergent builders. U.S. Pat. No. 4,663,071 discloses TMS/TDS in combination and individually and provides a method of preparation thereof. Although it is a stated object of the invention to provide such builder compositions and components thereof which can serve as especially effective builder materials in both granular and liquid detergent and laundry additive compositions, there is no mention of the particular problems in obtaining crystalline TMS/TDS for a dry composition and no method by which to solve this problem.
U.S. Pat. No. 4,798,907 discloses a method to prepare high yields of 2,2'-oxodisuccinate useful as a laundry detergent builder (referred to herein as malic monosuccinate or MMS). In a related application, EP 0 320 213, it is stated that the reaction yields an unusual, typically granular solid form of the divalent metal malate salt (emphasis added), acknowledging the problems with producing the compound in a solid form.
WO 91/19779 discloses a process for producing agglomerated 2,2'-oxodisuccinate/zeolite. This agglomerate is provided to overcome the difficulties of drying 2,2'-oxodisuccinate detergent builder salts which are often hydroscopic and therefore difficult to use is granular detergent formulations. Again, this application points to the problems in providing dry granular forms of such detergent builders.
This invention provides a method to prepare a dry particulate granular formulation which will enable the addition of TMS/TDS, and other detergent builders as are known to those of skill in the art, to dry detergents in conjunction with citric acid or salts thereof.